1. Field of the Invention
The present invention relates to a communication apparatus, and particularly to a communication apparatus which finds and uses a suitable path for routing a call connection.
2. Description of the Related Art
Asynchronous Transfer Mode (ATM) is a promising technology for multimedia data communications, which offers a range of transmission rates and different levels of quality of service to efficiently handle various types of I/O traffic over a single network medium, including data, audio, and video information. In many ATM-based enterprise networks, the Private Network-Network Interface (PNNI) protocol is used as the interface specifications for establishing Virtual Channels (VCs) between peer nodes. PNNI is a routing and signaling protocol which applies to a network of ATM nodes, or switches. In recent years, a connection technique known as the Soft Permanent VC (SPVC) has become practical in PNNI networks, by which permanent connections are used to attach end-systems to the network while switched connections are used to interconnect intermediate ATM nodes.
FIG. 12 shows an example of a PNNI network that supports SPVC service. In this PNNI network, five ATM nodes 301 to 305 are linked in a ring topology, and two end-systems 301a and 301b are attached to the left-most ATM node 301. Further, two more end-systems 302a and 304a are coupled to other ATM nodes 302 and 304, respectively. With the SPVC service, the end-systems 301a, 301b, 302a, and 304a are linked to their respective local ATM nodes with permanent VCs (PVCs), while network connections among the ATM nodes 301 to 305 will be established with switched VCs (SVCs).
Assume here that one end-system 301b attempts to communicate with a remote end-system 304a, while the ATM node 305 has not yet started up. It is also assumed that there is an ongoing session between the end-systems 301a and 302a over the link K. The calling end-system 301b now tries to set up a connection to the destination end-system 304a. This process starts with a best path selection at the ATM node 301 to which the calling end-system 301a is attached. In the example of FIG. 12, the path r1 is the preferred route at present, since the ATM node 305 has not yet started up. A call setup message is then sent from the source-side ATM node 301 to the destination-side ATM node 304 to establish an SVC connection between those two nodes. When this connection is successfully established, the end-system 301b can communicate with the remote end-system 304a. 
The above-described conventional routing mechanism, however, lacks flexibility in adapting to more suitable transit paths that would possibly emerge in the future. In the example of FIG. 12, a new path r2 between the two end-systems 301b and 304a will be formed when the ATM node 305 becomes operable. This new path r2 is better than the current path r1 in terms of the usage of network resources. In an attempt to migrate the route from r1 to r2, the conventional ATM node 301 would block its link K to temporarily close the connections to the neighboring ATM node 302. After unblocking the link K, the ATM node 301 re-executes a routing process, thereby selecting the new best path r2 for the intended connection. The link K, however, has been providing not only the connection to the end-system 304, but also the connection between the end-systems 301a and 302a. The blocking of the link K adversely affects this existing connection, causing a disruption of communication between the two end-systems 301a and 302a. 
As seen from the above example, the decision of which route to choose for a particular connection is affected by the growth or scale-down of a network, as well as the failure and recovery of network links and nodes. The conventional routing system has a problem in that the route optimization processing for a certain connection would cause an adverse effect on other existing connections, because of its disruptive blocking and unblocking operations that would reset all links attached to the source-side ATM node.